The present invention relates generally to computed tomography (CT) imaging systems. More particularly, the present invention relates to a system for sealing and cooling a rotating anode and associated vacuum vessel.
A CT imaging system typically includes a gantry that rotates at various speeds in order to create a 360° image. The gantry contains an x-ray source, such as an x-ray tube that generates x-rays by bombardment of an anode by a high energy electron beam from a cathode physically separated from the anode by a vacuum gap. The anode has a target that is coupled to a shaft, which rotates on a pair of anode bearings. X-rays are emitted from the target and are projected in the form of a fan-shaped beam, which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile for the generation of an image.
It is desirable to increase gantry rotating speeds and x-ray tube peak operating power such that faster imaging times and improved image quality can be provided. With increased gantry rotational speed comes increased load on the x-ray tube bearing. Although the use of bearing grease may allow for increased load on the bearing, since the bearing is inside the high voltage vacuum of the x-ray tube, grease or oil lubricated bearings cannot be utilized. Outgassing from the grease or oil leads to the degradation of the high voltage vacuum. This degradation causes high voltage instability and improper operation of the x-ray tube and can render the x-ray tube inoperable. Also, the use of silver or lead as a lubrication on the bearings is no longer able to sustain the required loads for adequate x-ray tube life.
Current tubes have an insert enclosed within a casing. The interior of the insert is under a high vacuum. An oil bath resides between the insert and the casing. The oil bath is utilized to cool the insert. Thermal energy radiates from a rotating anode within the insert, through the insert, and into the oil bath. The heated oil is cooled by circulation thereof through a heat exchanger. Thermal energy in the oil is transferred in the heat exchanger to ambient air, or, alternatively, a coolant, which circulates to and from an external chiller. This method of cooling the rotating anode in and of itself is also inadequate for increased gantry rotating speeds.
One design that currently exists for improved bearing performance and increased bearing life as well as improved cooling includes the use of a rotating insert, often referred to as a “rotating frame tube”. The rotating insert resides on a shaft that rotates on a set of fluid lubricated journal bearings or ball bearings. The ball bearings are cooled by an oil bath surrounding the insert. A rotating anode is located within and is formed or coupled as part of the insert. The rotating anode is directly cooled via a coolant circulating within the anode. Although the design provides increased bearing performance and operating life and direct cooling of a rotating anode, the design has several associated disadvantages.
The rotating frame tube design is limited in peak power and requires a large motor for rotation of the insert, which increases heat generation into a gantry and limits the x-ray tube thermal performance. The design also has a long electron beam path between the cathode and the target of the anode. The use of this long beam path can result in focal spot irregularities. These irregularities include a highly non-uniform intensity or unstable focus of the x-ray beam. The irregularities increase with an increase in target size and negatively affect image clarity and usefulness.
Thus, there exists a need for an improved x-ray tube having improved bearing performance and operating life and improved thermal performance without the above-stated disadvantages.